More precisely, the present invention relates to the problem of heterogeneous deformations that appear during transfer of a layer from a substrate termed the “donor substrate” onto a final substrate termed the “receiving substrate.” Such deformations have been observed in particular with the three-dimensional component integration technique (3-D integration) that requires transfer of one or more layers of microcomponents onto a final support substrate, and also with the transfer of circuits or with the fabrication of back-lit imagers. The transferred layer or layers include microcomponents (electronic, optoelectronic, etc.) produced at least partially on an initial substrate, said layers then being stacked on a final substrate that may optionally itself include components. Primarily because of the very small size and the large number of microcomponents present on a single layer, each transferred layer must be positioned on the final substrate with high precision in order to come into very strict alignment with the subjacent layer. Further, it may be necessary to carry out treatments on the layer after it has been transferred, for example to form other microcomponents, to uncover the surface of the microcomponents, to produce interconnections, etc.
However, the applicant has observed that after transfer, there are circumstances when it is very difficult, if not impossible, to form additional microcomponents that are aligned with the microcomponents formed before transfer.
This misalignment phenomenon is described with reference to FIGS. 1A to 1E that illustrate an exemplary embodiment of a three-dimensional structure comprising transfer, onto a final substrate, of a layer of microcomponents formed on an initial substrate, and formation of an additional layer of microcomponents on the exposed face of the initial substrate after bonding. FIGS. 1A and 1B illustrate an initial substrate 10 on which a first series of microcomponents 11 is formed. The microcomponents 11 are formed by photolithography using a mask that can define pattern formation zones corresponding to the microcomponents 11 to be produced.
As can be seen in FIG. 1C, the face of the initial substrate 10 comprising the microcomponents 11 is then brought into intimate contact with one face of a final substrate 20. Bonding between the initial substrate 10 and the final substrate 20 is generally carried out by molecular bonding. Thus, a buried layer of microcomponents 11 is formed at the bonding interface between substrates 10 and 20. After bonding and as can be seen in FIG. 1D, the initial substrate 10 is thinned in order to remove a portion of the material present above the layer of microcomponents 11. A composite structure 30 is thus formed from the final substrate 20 and a layer 10a corresponding to the remaining portion of the initial substrate 10.
As can be seen in FIG. 1E, the next step in producing the three-dimensional structure consists of forming a second layer of microcomponents 121-129 at the exposed surface of the thinned initial substrate 10 or of carrying out additional technical steps on that exposed surface in alignment with the components included in the layer 10a (contact points, interconnections, etc.). For the purposes of simplification, the term “microcomponents” is used in the remainder of this text to define devices or any other patterns resulting from technical steps carried out on or in the layers that must be positioned with precision. Thus, they may be active or passive components, a mere contact point, or interconnections.
In order to form the microcomponents 121-129 in alignment with the buried microcomponents 11, a photolithography mask is used that is similar to that used to form the microcomponents 11. The transferred layers, like the layer 10a, typically include marks both at the microcomponents and at the section forming the layer that are in particular used by the positioning and alignment tools during the technical treatment steps, such as those carried out during photolithography.
However, even using positioning tools, offsets occur between some of the microcomponents 111-119 and 121-129 , such as the offsets Δ11, Δ22, Δ33, Δ44 indicated in FIG. 1E (respectively corresponding to the offsets observed between the pairs of microcomponents 111/121, 112/122, 113/123 and 114/124).
Such offsets are not the result of elementary transformations (translation, rotation or combinations thereof) that could originate in inaccurate assembly of the substrates. These offsets result from non-homogeneous deformations that appear in the layer derived from the initial substrate while it is being assembled with the final substrate. In fact, such deformations involve local and non-uniform displacements of certain microcomponents 11. In addition, certain of the microcomponents 121-129 formed on the exposed surface of the substrate after transfer exhibit positional variations with those microcomponents 11 that may be of the order of several hundred nanometers, or even of micrometer order.
This phenomenon of misalignment (also termed “overlay”) between the two layers of microcomponents 11 and 121-129 may be a source of short circuits, distortions in the stack, or connection defects between the microcomponents of the two layers. Thus, when the transferred microcomponents are imagers made up of pixels, and the post-transfer treatment steps are aimed at forming color filters on each of those pixels, a loss of the colorizing function is observed for certain of those pixels.
The overlay phenomenon thus results in a reduction in the quality and the value of the fabricated multilayer semiconductor wafers. The impact of the phenomenon is increasing because of the ever-increasing demand for miniaturization of microcomponents and their increased integration density per layer.
Problems with alignment during fabrication of three-dimensional structures are well known. The document by Burns et al., “A Wafer-Scale 3-D Circuit Integration Technology,” IEEE Transactions on Electronic Devices, vol. 53, No. 10, Oct. 2006, describes a method of detecting variations in alignment between bonded substrates. The document by Haisma et al., “Silicon-Wafer Fabrication and (Potential) Applications of Direct-Bonded Silicon,” Philips Journal of Research, vol. 49, Nos. 1 and 2, 1995, emphasizes the importance of wafer flatness, in particular during polishing steps, in order to obtain good quality final wafers, i.e., with as few offsets as possible between the microcomponents. However, those documents are concerned only with the problem of positioning the wafers while they are being assembled. As explained above, the applicant has observed that even when the two wafers are perfectly mutually aligned when put into contact (using marks provided for that purpose), non-homogeneous displacements of certain microcomponents occur following initiation of the bonding wave.